Thermoregulation is an integrated system of biological mechanisms, responsible for maintaining an almost constant internal temperature regardless of the climatic conditions outside the organism. These mechanisms - particularly effective in birds and mammals (all homeothermic animals), less so in fish, amphibians and reptiles (poikilothermic animals) - include processes of production, storage And dispersion of heat.
Since the obese subject frequently does not eat abnormally when compared to other normal-weight individuals, who sometimes even eat more, it is presumable that - with the same physical activity - the alterations of the thermoregulatory processes can lead to a reduced energy consumption, with accumulation of excess energy in the form of fat. Thin subjects, unlike the obese, would therefore be better at disposing of excess food (see brown adipose tissue) in the form of heat.
Thermoregulation can first and foremost be voluntary or involuntary. In the first case it is the animal itself that voluntarily sets in motion adequate behavioral strategies, such as the search for a den sheltered from the elements or the migration to the places most suited to maintaining its own body temperature.
Even involuntary thermoregulatory responses can be evoked by exposure to cold or hot environments. In any case, they involve the intervention of the hypothalamic thermoregulatory center, capable of capturing and processing the signals coming from the cutaneous and central thermoreceptors (located in the brain, spinal cord and central organs), coordinating the physiological response most suitable for maintaining body temperature.
Thermoregulation in cold environments
The thermoregulatory adaptations to cold have the purpose of conserving and / or producing heat.
The ability of an organism to produce heat is called thermogenesis; it is largely mandatory and linked to the physiological and metabolic processes responsible for movement, digestion, absorption and processing of the nutrients introduced with the diet.
Mammals have the ability to increase heat production (optional thermogenesis), whether or not it involves the thrill mechanism. In the first case we speak of shivering thermogenesis. This mechanism leads to the production of heat through a rhythmic and isometric contraction of muscle tissue, not aimed at movement. The alternation of contractions and relaxation leads to a characteristic tremor called shiver, which appears when the body temperature tends to decrease "noticeably". The shiver generates an amount of heat up to 6-8 times greater than that produced by the resting muscle. Typically , it occurs only when maximal vasoconstriction (see below) has been unable to maintain body temperature.
Non-thrill thermogenesis, also called chemical thermogenesis, involves the production of heat through exothermic (heat-generating) biochemical reactions. These reactions occur in particular organs, such as brown adipose tissue (BAT), liver and muscle.
The brown adipose tissue, typical of hibernating animals and scarce in humans (greater in infants), is thus defined for the characteristic brown pigmentation (visible to the naked eye) given by the carotenoids present at the mitochondrial level. These energy centers of the brown adipose cell are distinguish for a further characteristic, the presence of the mitochondrial protein UCP1. This protein, located at the level of the mitochondrial membrane, has the characteristic of decoupling oxidative phosphorylation, thus favoring the production of heat at the expense of the formation of ATP molecules. , the brown adipose tissue has the purpose of burning nutrients (mainly fat) in order to increase the production of heat. The activation of the brown adipose tissue, stimulated by cold, is mainly linked to the release of norepinephrine and its interaction with the β3 receptors, but also guaranteed by endocrine mechanisms such as the release of T3 e T4 from the thyroid. The greatest deposits of brown adipose tissue are recorded in the interscapular, periaortic and perirenal area; at these levels, they are located near blood vessels, to which they release heat so that it is transported with the blood flow to the peripheral areas of the body.
It is currently believed that the liver also participates in thermoregulation, increasing its metabolic activity - resulting in the production of heat - when the human body is exposed to low temperatures. Another recent discovery was the discovery of isoforms of the UCP1 protein in muscle, which suggests an alleged thermogenetic role of metabolic origin (in addition to the ability to produce heat through shivering). Finally, "exposure to low temperatures increases the" cardiac activity, necessary to support the metabolic demands of the active tissues in these circumstances (such as BAT) and to increase the transport of the heat produced therein in all anatomical districts. In addition to guaranteeing all this, the increase in cardiac activity is in itself capable of producing a non-negligible amount of heat.
The control of heat losses is governed by the physical laws of conduction, convection, radiation and evaporation.
CONDUCTION: heat transfer between two objects at different temperatures, in contact with each other through a surface.
RADIATION or RADIATION: heat transfer between two objects at different temperatures, which are NOT in contact. The loss or acquisition of heat occurs in the form of radiation with wavelengths in the visible or infrared range; to be clear, it is the same way by which the sun heats the earth through space. constitutes over half of the amount of heat lost by the human body.
CONVECTION: transfer of heat from a body to a source that moves through it (currents of air or water). The movement of water or cold air through the warmer skin causes the continuous elimination of heat.
EVAPORATION: heat transfer by passage from the liquid to the gaseous state of the fluids lost through sweating, insensitive losses through the skin and respiratory tract.
The reduction of heat dispersion in the environment occurs essentially through the containment of the cutaneous blood flow (vasoconstriction) and the piloerection (in fur animals, between the warm skin and the cold environment, an air cushion is created which acts as a thermal insulator).
The increase in appetite, for its part, increases the production of heat through the thermogenetic mechanisms induced by the diet, and supports the energy demands of the thermogenetic organs.
Thermoregulation in hot environments
During the stay in warm environments the organism reacts through a series of thermodispersive mechanisms, in many ways contrary to those just illustrated; moreover, there is the suspension of the metabolic processes underlying the optional thermogenesis. Among these we remember the cutaneous vasodilation and the increase of sweating, frequency and depth of breath (polypnea), all processes that aim to increase the dispersion of heat through evaporation. In these circumstances, appetite and heart rate also decrease, in response to a lower demand for oxygen by the thermogenetic organs.
Among the long-term adaptation processes it is also possible to appreciate a decrease in the pituitary secretion of thyrotropic hormone, with a consequent slowdown of the metabolism, therefore of the production of heat.
As mentioned in the previous chapter, the vasoconstriction process is largely controlled by the sympathetic nervous system. The smooth muscle in the precapillary sphincters and arterioles receives input from postganglionic sympathetic (adrenergic) neurons. If the deep temperature drops (exposure to cold), the hypothalamus selectively activates these neurons, which through the release of noradrenaline determine the contraction of the arteriolar smooth muscle, reducing the cutaneous blood flow. This thermoregulatory response keeps the blood warmer to the internal organs. , minimizing the blood flow on the skin surface made cold by the weather. While vasoconstriction is an active process, vasodilation is a predominantly passive process, which depends on the suspension of vasoconstrictor activity by inhibiting sympathetic activity. If this process is typical of sympathetic activity. body extremities, in other parts of the body vasodilation is favored by specialized neurons that secrete acetylcholine. Special cases are also represented by the local dilation of some vascular districts following the release of nitrogen monoxide (NO), or other vasodilating paracrine substances.
In the context of thermoregulation, the cutaneous blood flow varies from values close to zero, when it is necessary to conserve heat, up to almost 1/3 of the cardiac output when the heat must be released into the environment.
